This study examines the kinetics of the longest lasting form of short-term depression at excitatory hippocampal synapses. After initial depletion of the readily releasable pool (RRP), continued 20 Hz stimulation was found to be fast enough to drive presynaptic neurotransmitter exocytosis maximally; maximal is defined here as the rate needed to maintain the RRP in a near-empty steady state. Induction of depression proceeded in two distinct phases. The first was caused by RRP depletion, while the second is shown to reflect the progressive reduction of the overall rate at which new vesicles are supplied to the RRP, and is termed "supply-rate depression." Supply-rate depression is identified further with the emergence, during heavy use, of a rate-limiting vesicle trafficking step that slows the timing of RRP replenishment by switching from a fast (t = ~ 7 s) to a slow (t = ~ 1 min) vesicle supply mechanism. Both mechanisms apparently follow first-order kinetics. After the induction of the maximum amount of depression, individual synapses were only able to output less than 1 quantum of neurotransmitter per synapse per second, matching previous predictions based upon cell biological measurements of synaptic vesicle cycling. Surprisingly, the onset of supply-rate depression occurred with a marked delay, not having a detectable impact on synaptic function until after several seconds of continuous use. The delayed onset is not consistent with traditional vesicle trafficking models, but may be important for limiting the impact of supply-rate depression to pathological episodes, and might function as a native anti-epilepsy device.